In 3GPP (Third Generation Partnership Project), according to a radio communication system corresponding to LTE (Long Term Evolution) having a standard being currently designed, a radio base station eNB performs assignment of a radio resource in radio communication between the radio base station eNB and a radio terminal UE (for example, refer to Non Patent Literature 1). Furthermore, in the radio communication system corresponding to the LTE, one of frequency division duplex (FDD) and time division duplex (TDD) is employed in the radio communication between the radio base station eNB and the radio terminal UE.
Moreover, in an LTE (TDD-LTE) radio communication system employing the TDD, there has been discussed a feature where a radio base station eNB performs control for adaptively directing a beam (adaptive array control) toward the radio terminal UE at the time of transmission of a downlink radio signal, in order to ensure communication quality between the radio base station eNB and a radio terminal UE that is moving.
Firstly, according to a technique of calculating an antenna weight, when a radio base station eNB receives a sounding reference signal (SRS) which is an uplink radio signal from a radio terminal UE, it is considered that the radio base station eNB assigns a downlink radio resource (a downlink resource block), which has a frequency band equal to a frequency band of the latest received SRS, to a radio terminal UE serving as a transmission source of the latest received SRS, and further calculates an antenna weight for the assigned downlink resource block.
However, in the aforementioned technique, each radio terminal UE independently sets a transmission timing of SRS. Accordingly, a frequency band that is used in the transmission of the SRS may be concentrated on a specific frequency band. In such a case, although there is a vacancy in the downlink resource block, it is probable that use efficiency may be deteriorated because a downlink resource block of a specific frequency band is concentrated on a plurality of radio terminals UE.
Secondly, according to a technique of calculating the antenna weight, when a radio base station eNB received a sounding reference signal (SRS) which is an uplink radio signal from a radio terminal UE, it is considered that the radio base station eNB calculates an antenna weight for downlink RB which has a frequency band equal to a frequency band of the SRS.
However, in the TDD-LTE radio communication system, OFDMA (Orthogonal Frequency Division Multiplexing Access) is employed in downlink radio communication and SC-FDMA (Single Carrier Frequency Division Multiple Access) is employed in uplink radio communication. In these multiplexing schemes, individually in uplink and downlink, a radio resource (a resource block: RB) is arranged in two-dimensions of a frequency and a time to realize user multiplexing.
Therefore, there is a case in which a frequency band of downlink RB assigned to a radio terminal UE is not equal to a frequency band of uplink RB. In such a case, since there is no downlink RB having a frequency band equal to a frequency band of SRS, it is not possible to calculate an antenna weight for the downlink RB.
Thirdly, according to a technique of calculating the antenna weight, when a radio base station eNB receives a sounding reference signal (SRS) which is an uplink radio signal from a radio terminal UE, it is considered that the radio base station eNB assigns a downlink radio resource (a downlink resource block), which has a frequency band equal to a frequency band of the latest received SRS, to a radio terminal UE serving as a transmission source of the latest received SRS, and further calculates an antenna weight for the assigned downlink resource block.
However, in the aforementioned techniques, in order to transmit the SRS, each radio terminal UE uses an entire frequency band in which a bandwidth is determined in advance. As described above, when a bandwidth of a transmission frequency band of the SRS is fixed, if interference from another radio base station eNB occurs in the transmission frequency band of the SRS, it is not possible to appropriately cope with the interference.